Tuesday, January 31, we fired a second test burn with our new device. An exciting afternoon with Brad Rush at Four Oaks Farm in north Topeka, learning about controlled combustion, uncontrolled combustion and meeting the neighborhood firemen. To convert a large brush pile into biochar, we fabricated a TLUD from a 55-gallon barrel and one length of stovepipe.
Before we fired up our second burn, we made a few simple upgrades to the equipment. In our first burn on Friday, we learned about a few weak features in our equipment, plus we have several improvements to design, build and test.
First, more air intake is needed at the burn barrel bottom than the 1-inch and 2-inch bung holes. Especially we need an air vent in the center to suck air up through the center of the biomass. So, Brad cut a 3-inch square hole in the center of the bottom.
|Hear the TLUD Dragon roar!
cooking with pyrolysis gas
Second, more air intake is needed at the base of the chimney to mix with pyrolysis gases exiting the burn barrel. Brad cut four 2x3-inch slots on opposite sides near the bottom of the stovepipe.
Air vented into the chimney will swirl with hot gases from the biomass combustion in the barrel below. With adequate air-to-gas ratio, they ignite to create a secondary fire in the chimney. This gas fire is high temperature, and thus should be smokeless, with no soot or sparks.
6-inch Fiberglass Insulation
Third, a 6-inch layer of pink fiberglass insulation. We cut and stacked two rings of fiberglass around the barrel. We found a "tomato cage" ring of wire fence to hold the thick fiberglass blanket in place.
This creates a thermal container to hold heat in the barrel, and curtail radiated heat. This will significantly raise TLUD temperature, and make our work area comfortable and safer. A hotter burn means more complete combustion, and thus, smokeless and more efficient.
A high temperature burn will melt and degrade fiberglass, but this will demonstrate the principle and benefits of an insulated burner. Eventually, we'll buy ceramic fiber insulation for maximum thermal efficiency up to 1500 degrees.
Outer Wind Shield
Fourth, we added a second length of stovepipe to create a 4-foot chimney. This should sharply increase the updraft, and suck more air in the bottom air vents and up through the biomass. A third length can further improve draft, but then we'll need a stepladder to place the chimney cap. Insulating the chimney can also incease natural updraft.
Steady, strong Kansas winds will play with an updraft chimney. I've seen wind snuff out a gas flare, or create such strong downdraft that smoke blows out the barrel bottom.
So, we installed two wind shields around the base of the chimney to reduce suction from negative wind pressures. If needed, we can cover or screen the open top of the outer windshield to sharply curtail caprious wind gusts.
|Double Wind Shield
In the photo at right, drilled holes and cut slots for the chimney air vents are hidden behind and below the inner wind shield. This proved to be a reallly smart move, but we didn't plan for this—we were fortunate to find two pieces of scrap metal, just the right size and shape, lying on the ground at Four Oaks.
Brad again used mineral spirits and wood pellets to ignite and accelerate the burn. The intention is to get a large, hot fire started quickly in the top of the barrel. We carefully spread spirits-soaked pellets evenly across the top of the twigs, so the flame front will maintain symmetry as it burns down through the biomass
One kitchen match, and in seconds, fire quickly danced up out of the barrel. Twig ignition was rapid and vigorous. In less than a minute, bright orange flames twisted two feet above the chimney hole. Brad slid the chimney into position, and pressed pink fiberglass in the space between windshields, but the intense heat made his simple task a challenge.
In three minutes, red flames shot out the top of the chimney, along with thin wisps of smoke. I couldn't determine if this was wood smoke, or paint finish burning off metal. Whatever the source, the wisps soon vanished, and the rest of the burn was near-perfect smokeless, despite a devilish wind.
2nd Burn Begins
Brad commented on our dilemma that once we fire up a burn, we have no quick way to shut it off. Our best hope for fast shutdown is to choke off underground air intake and slowly suffocate the fire. Second step is to open the top to slowly spray in a few gallons of water.
Wind shields were effective to moderate wind, stabilize air flow, and contain and shelter flames. The chimney cap created enough shielded space for flames to exit the chimney cleanly, even in strong wind. Slanted slots in the chimney cap deflect gusts enough to soften the winds' suction and create a sheltered space for flaming gases to exit the stovepipe. I saw little likelihood a wind gust can snuff out our chimney fire, or create serious backdraft.
The burner blazed like a charm—better than expected. An 8-inch column of flame roared up the 4-foot chimney to form a huge, boiling gas flare under the chimney cap. Despite stiff winds, this 8-inch gas flare flickered and blew up to 18 inches beyond the chimney cap.
|Placing the Chimney Cap
I expected this intial outburst to soften as the flame front dropped a few inches down into the barrel. To my surprise, the noisy, large gas flare continued relentlessly for a long time, hardly slackening. This blaze roared in a stiff wind at least 45 minutes—nearly a full hour.
Air vents and chimney performed beyond expectations, with a strong up-draft. Our biochar stove vigorously sucked air in the bottom, spewed flaming gas out the top, with nearly no smoke, radiating intense BTUs of heat. Insulating the first length of chimney will further boost the up-draft. An electric blower to force air through the biomass seems unnecessary, although one may still be useful to accelerate a burn.
A TLUD is a 2-stage burner. The primary fire burns biomass, slowly descending from top to bottom inside the 55-gallon barrel. The primary fire burns slowly down thru packed biomass, steadily eating its way toward new fuel and the source of oxygen.
A second combustion zone is created in the chimney by burning off-gases from the smoldering primary fire—mostly hydrogen, methane, carbon monoxide—all flammable.
Without oxygen, there is no combustion. Oxygen in air flowing up the barrel is consumed by flames in the burn zone, and not enough oxygen is present to completely burn the biomass. Inadequate oxygen at the burn zone means wood can't completely burn to ash, but is only gasified—heated enough to boil out volatile vapors and lightweight chemicals.
Burn 60% Complete
Carbon burns at the highest temperature, so it burns last. Thus, most carbon remains unoxidized, and is left behind as the flame front travels down. This carbon residue is charcoal. Embedded in this carbon matrix are the dense tars and resins plus the mineral ions.
Inadequate oxygen also means pyrolysis gas released from biomass isn't completely burned. Super-heated pyrolsis gas rises unburned into the chimney, where air is sucked in through shielded holes around the chimney base. This gas and air mixture can now ignite, and its flaming explosion acccelerates the upward movement. A negative pressure created sucks more gases up out of the barrel, pulls more air in the barrel bottom vents.
Even with insulation, heat radiated by the barrel was potent. Inner areas of fiberglass melted, but less than I expected. Near the stovepipe, I could only shoot brief videos of the flaming chimney cap before intense heat forced me to pull my hand away.
We followed the slow descent of the flame front in the barrel by a progressive discoloration of the fiberglass. Pink became brown as heat scorches the new insulation. At right, the burn zone is clearly visible. Counting fence squares, this burn is 3/5ths (= 60%) done.
Strong draft plus heat-containing insulation yield a high temperature fire to assure complete combustion, with no smoke, soot or creosote, except a few wisps in the first five minutes.
Watching the flames, feeling the intense BTUs released by controlled combustion of 50 gallons of loosely packed biomass makes a profound impression. A question on every mind is: "How do we capture that energy, rather than burn it off right away?"
|Burnt Barrel and Biochar
A first step is to design a burner to suck air through it to function is a space heater. More important is to add a heat exchanger on and in the chimney to make and store hot water for radiant heat systems. Third step is to design a water-cooled condenser to collect "wood vinegar" and other hydrocarbons to refine into liquid biofuels.
In the end, we had ten gallons of charred twigs. Larger, denser stems were only surface charred, so we raked them out to run through the third burn. Screening left perhaps three gallons of rather fine textured biochar, and another five gallons of larger well-charred stems and stubble that needs crushing or grinding.
We're satisfied by the imperfect successes of the second burn. Our simple equipment functions beyond expectations, and this method to make biochar looks easy, reliable, flashy to show in public, a snap to teach others. And we have still further refinements of equipment and methods yet to implement.
Unfortunately, as we disassembled the chimney to open the barrel and examine our char, Brad dropped a hot chimney pipe on winter drought dry grass. Behind him, hot metal ignited dry grass like fluffy tinder, and ankle-high flames lept up.
As I shoveled dirt from around the TLUD, I looked over to see a grassfire behind Brad, already well underway, driven east by steady, strong wind. Quickly, the fire was off, whipped by wind across the field.
Fortunately, we almost beat the fire out with shovels.
Unfortunately, wind whipped flames out of our work area.
Fortunately, on the north, is a wide plowed field.
Unfortunately, wind was blowing southeast.
Fortunately, downwind was a metal recycling yard.
Unfortunately, the place is bordered by trees and bush
with lots of dry leaves and deadwood.
Fortunately, Brad had his cell to call Four Oaks farmer Art.
Unfortunately, Art didn't answer.
Fortunately, Brad called 911.
Unfortunately, they didn't know where Gordon Street is,
and Brad didn't know the farm's street number.
Fortunately, the Fire Station is around the corner.
Unfortunately, the pumper truck was already out on call.
So, we were on our own for quite a while—two 60-year-old men beating down flames with shovels. I took the south flank. Brad went east to the head of this flying flame dragon.
I found it easy to smother the thin edge of flames. Somehow, Brad and a shift in wind turned the fire south, away from the tinderbox of trees and toward Gordon Street.
Eventually, a firetruck pulled up on Gordon. A fireman with sprayer and foam bottle hit the north flank. Soon firemen had hoses off the truck to spray the leading edge, then my south flank.
Meanwhile, wind subsided and fire stopped running. Then, the west edge traveled west to burn fiercely in tall grass around the power pole and irrigation pump, and about to burn up under Brad's truck.
My first experience with a legendary Kansas grassfire. Hopefully, my last. Later, in a quiet moment, I wondered what a tallgrass prairie fire must be like. And given how dry and this warm winter is, I fear how hot and fiery this summer will likely be.
In any event, our second experiment in controlled combustion went superb. We made a respectable batch of fine textured, easy-to-crumble biochar. And when the hot chimney hit the dry grass, we unexpectedly made an extra half-acre of biochar outside the burn barrel.
|Half Acre of Biochar